This invention relates to an improved prosthesis interface surface and the method of implanting it in conjunction with an artificial joint replacement procedure. At present, the majority of total joint components are stabilized with methyl methacrylate bone cement which functions as a mechanical interlock between the associated bone and the replacement joint. Most of these total replacement prostheses have been for hip and knee joints and the designs utilized involve combinations of plates, stems and/or peg structures to achieve fixation. Since the replacement joints have little or no inherent stability without cement, the usual insertion procedures include production of oversized holes and filling the holes with cement during implantation. Several disadvantages of implants which require cement have been documented as follows:
(1) The single most frequent failure mode of cemented implants has involved the loosening and subsequent failure thereof. While the rate of loosening varies with component design and surgical techniques, loosening has universally been reported to be a major limiting factor to long term success of the implant.
(2) The major disadvantages with cemented implant designs are: loosening or fragmentation at the bone-cement or prosthesis-cement interface; difficulty in removing cement in affected patients or in previous replacement failures that need revision; poor mechanical properties of methyl methacrylate cement in that while it resists compression well, it responds poorly to torsion, shear and tension; susceptibility of blood, fluids or air to be trapped in the cement to thereby cause stress risers and thus reduce fatigue strength; difficulty in controlling penetration of the cement--while recent work has shown that pressurizing the cement improves fixation to trabecular bone, the preparation of a clean trabecular bone surface is critical and is difficult to achieve; the enlarged opening required for cement insertion requires the sacrifice of a substantial amount of trabecular bone; and the physiological affects of methyl methacrylate cement which in operation is a thermo setting acrylic are still as yet unknown.
A second class of prosthesis interface is designed to be used with no associated cement. This class includes several hemiarthroplasties of the hip as well as the total knee tibial component which comprises a polyethylene structure with two large pegs. The pegs are driven into undersized holes and include fins thereon which are designed to provide stable fixation of the implant. The main draw back to these interference fit type designs is the fact that they utilize large protrusions, such as, stems or pegs which may concentrate stress in some areas and cause stress relief in others. Bone remodeling responses to these stress patterns may cause disadvantages bone substance reorientation and subsequent failure of both the bone and the implant.
A further and more recent class of implants have been developed which include a porous surface coating made from porous metal, polyethylene or ceramic materials. These implants have been used as hip and knee joint replacements as well as in conjunction with fixture of dentures. Basically, this class of devices has similar design to that of cemented implants except that a porous material is used at the interface. While the porous implants have not been use long enough to provide good statistical evaluations, several disadvantages have become evident recently:
(1) Bony ingrowth into the surface of the prosthesis has been reported to occur only if the implants are extremely stable and secured for three to eight weeks. In situations where movement of the components has occurred, a fibrous interface may form on the bone and thus cause a loose component. If this occurs, post-operative patient mobility may be severely compromised.
(2) The prosthesis as presently designed needs an extremely accurate initial fit and as such require tedious and sometimes difficult surgical techniques to achieve such a fit.
(3) The geometries of present implant designs do not optimize or maximize the potential porous surfaces and thus limit the amount of bony ingrowth to a limited surface area which reduces the strength of the bond between the implant and the associated bone.
A further group of components include designs which utilize screws or staples to augment the fixation thereof. Additionally, artificial ligaments have been designed which contain metallic bony insertions. These components have disadvantages as requiring accuracy in fabricating the screw holes or the receiving surfaces for the staples and further the screws may act to crack the bone tissue.
Applicants are aware of the following prior art:
U.S. Pat. No. 2,910,978 to Urist discloses a hip socket having a plurality of anchoring spikes 19 which project the exterior bottom of the shell 15. The spikes 19 provide a simple interference fit as the sole means for maintaining the socket in position. The device of Urist is different from applicants' invention because, firstly, in applicants' invention, the interference fit is only used on a temporary basis while the bony growth is forming; secondly, the device of the present invention provides a contact stress spectrum or gradient to assure an optimum ingrowth environment. This contact stress gradient is a design feature determinate for the shape, size and array of cones, a concept not confronted by Urist; thirdly, the device of the present invention is not limited to hip implants but rather is applicable for any type of prosthetic body implant.
U.S. Pat. No. 3,683,421 to Martinie discloses a prosthetic joint assembly wherein the bone attachment socket is provided with a plurality of apparently conical projections 62 (FIG. 4) which are designed to more firmly secure the socket to the associated acrylic cement. The patent to Martinie is obviously quite different to the teachings of the present invention since there is no disclosure therein of porous coatings, bone ingrowth, contact stress or sequence of interference in permanent ingrowth fixation.
U.S. Pat. No. 3,728,742 to Averill discloses a knee or elbow prosthesis including a bone contact surface which is provided with serrations 32 to assist in the firm anchorage by the associated cement. As is seen in FIG. 6 of Averill, the serrations are in no way related to the configuration of the surface of the present invention. Further, there is no disclosure in Averill of the concept of ingrowth of bony tissue.
U.S. Pat. No. 3,808,606 to Tronzo discloses a plurality of embodiments of bone implants wherein a porous exterior surface is provided to permit ingrowth of bone for secure fixation. As disclosed therein the porous exterior surface may be made of stainless steel, titanium or titanium alloys which may be applied as a powder and then sintered, if desired. The present invention differs from Tronzo in that (1) Tronzo does not disclose a configuration of conical projections like that of the present invention, (2) Tronzo does not treat a specific design for contact stress on the associated bones, (3) Tronzo does describe the use of an interference fit preliminary to ingrowth fixation but differs in the way that this is accomplished and (4) No provision is made for an optimal interface between the prosthesis and the associated bone.
U.S. Pat. No. 3,869,731 to Waugh, et al. discloses a tibial implant including concentric rings which bite into the tibial bone. There is no disclosure in this patent of ingrowth or a porous coating to facilitate bony ingrowth. The concentric rings as best shown in FIG. 7 bear no resemblance structurally or functionally to the teachings of the present invention.
U.S. Pat. No. 4,021,864 to Waugh discloses an ankle prosthesis including a tibial member provided with truncated pyramidal teeth 20 best shown in FIGS. 6 and 7 which enhance the retention in the bone in conjunction with a suitable bone cement. Obviously there is no concept disclosed in Waugh of the use of a porous prosthetic surface to facilitate the bony ingrowth nor any disclosure of the specific structure disclosed in the present invention.
U.S. Pat. No. 4,166,292 to Bokros discloses a stress reinforced artificial joint prosthesis made preferrably of graphite with a pyrolytic carbon coating. As shown, the prosthesis includes ridges 24 of triangular cross-section which are provided to enhance the permanent fixation of the prosthesis. It is believed that the use of the graphite substrate is more related to a concept of shock absorption or trying to match the prosthetic elastic modulus or stiffness to that of the bone. This concept in no way relates to ingrowth fixation, a spectrum of contact stress or any other concepts disclosed herein.
U.S. Pat. No. 4,231,120 to Day discloses an endoprosthetic orthopedic device having a method of securement involving an elongated stem with annular or helical fin-like elements extending radially outwardly therefrom. This device is inserted into a slightly undersized recess in the bone and the fins provide fixation therein. With regard to this patent, it is noted that no use of porous coated metal is described therein. True ingrowth into 100 to 400 micron pores is not used. The prosthetic component of Day is made of polyethylene and the fixation technique described therein involves defomation of this large polyethylene peg.
U.S. Pat. No. 4,309,777 to Patil discloses an artificial intervertebral disc employing spikes 18 and 22 on the upper and lower surfaces thereof for engagement with the respective vertabra. Springs 16 within the device provide the force necessary to insert the spikes in the vertebra. With regard to Patil, the above described projections are not coated with any porous material and rely solely upon interference fit for securement into the respective vertebra.
U.S. Pat. No. 4,355,429 to Mittelmeier, et al. discloses a slide prosthesis for the knee joint which is secured without cement. Anchoring pins are integrally provided and refer to by reference numerals 21 and 22 and have a profile-like out of a bone screw. These pins are inserted into slightly undersized holes drilled into the bone. If desired, the contact surface may be covered with a coating of bio-active particles. In this patent, the prosthesis is made of carbon fiber, reinforced plastics or aluminum oxide ceramic covered with bio-active particles. There is no disclosure of covering of the bone engaging surfaces of the prosthesis with a porous coating material. There is no disclosure therein of a specific design for a spectrum of contact stress nor is a sequence of interference giving away to durable ingrowth fixation described.